1. Field of the Invention
The present invention relates to a toner composition to be used for an image forming apparatus utilizing the electrophotographic method, and more particularly to a toner composition which is good in charge stability, fluidity and delivering ability to a developing apparatus, and to an image forming method utilizing the same.
2. Description of the Prior Art
An image forming apparatus utilizing the electrophotographic method is well known.
In the image forming apparatus utilizing the electrophotographic method, images are generally formed onto a sheet of copy paper through the following processes.
After uniformly charging a photoconductor which serves as an image-holding body, images are exposed onto the surface of the charged photoconductor. A latent image is formed by attenuating electrostatic charges during the exposure of light. Then the electrostatic latent images are visualized by developing with toner to form a toner image. The toner images are transferred onto a sheet of copy paper.
The above-described developing processes are classified into two categories; one is the two component development method using a developing agent in which a carrier composed of a magnetic material and a toner are mixed together, and the other is the single component method utilizing a toner only.
In the two component development method, an appropriate charge is imparted to the toner by mixing with the carrier. In the single component development method, on the other hand, a predetermined charge is given to toner by the friction among the toner particles, or by contact with friction-charging parts, such as the development sleeve, brush-cutoff regulating sleeve or toner press-adhering blade. As a result of this charging phenomena, the toner particles adhere onto the photoconductor by an electrostatic force acting between the charged toner particles and the electrostatic charge composing aforementioned electrostatic latent images, thereby forming visible images.
The two component development method is described referring to FIG. 5. The image forming apparatus used for the two component development method comprises a photoconductor 10, a developing apparatus 5 and a toner container 1. A stirring roller 7 to mix the developing agent D homogeneously, and a development sleeve 8 are disposed inside a developing apparatus 5. A development sleeve 8 is composed of a magnet or the like, in the vicinity of which a magnetic brush composed of a chain-like array of the carrier in the developing agent D is formed. Toner T adheres onto the carrier by frictional charging. The electrostatic latent images formed on the photoconductor 10 through the charging and exposure processes are developed by the above-described toner T. The toner image corresponding to the electrostatic latent image is transferred onto the copy material such as a sheet of copy paper, followed by fixing on the copy material by the fixing equipment such as a fixing roller (a heated roller or a press roller).
When the toner concentration in the developing agent D is decreased during the operation, the following system operates within the entire system to keep the predetermined toner concentration in the developing apparatus 5.
A magnetic sensor 9 to detect the toner concentration in the developing agent D is equipped in the developing apparatus 5, and the magnetic sensor 9 is linked with the driving part of the toner-supply mechanism 11 disposed to the lower part of the toner container 1. When the toner concentration in the developing agent D is decreased, the magnetic sensor 9 transfers a signal to the driving part of the toner-supply mechanism 11, thereby feeding down a predetermined amount of toner T from the toner container 1 to the developing apparatus 5 by rotating the toner-supply mechanism 11.
Good fluidity of the toner T is required to form an excellent quality of images in the two component development method. That is, if fluidity of the toner T in the developing apparatus 5 is insufficient, output of the magnetic sensor 9 will fluctuate and charging will be unstable, thereby causing image overlapping and scattering of the toner particles. Particularly, if a toner-supply mechanism 11 by which toner is supplied to the developing apparatus 5 from the remote position using spiral screw 2 as is shown in FIG. 1 is applied, efficiency of toner feed to the developing apparatus 5 will be decreased.
Addition of fine particles of silica has been suggested to improve the fluidity of the toner. Since fluidity of a toner composition including fine particles of silica is rather good, feeding efficiency of the toner composition from the toner container 1 to the developing apparatus 5 and charge stability of the toner composition can be improved.
In the above-described toner composition, however, fluidity of the composition is still unsatisfactory when micro-particles of the toner having the grain diameter of 10 .mu.m or less are used to realize a high quality of images, or when formation of the images is performed under the circumstances of high temperature and high humidity. Some problems arise in that feeding performance from the toner container 1 and charging stability decreased.
A laser printer, a LED printer and the like have been developed in recent years, wherein the image scanning part and the image output part are separated from each other. In these apparatuses, a reversed development method, i.e., the toner is made to adhere to the portions where the charge at the portions of the image exposure has been attenuated, is adopted (in contrast to the above, where toner is made to adhere to the portions where charge still remains in the normal development method).
In such a reversed development method, the images are exposed onto the photoconductor by a laser output of the digital processed image signals. In these methods, gradation is expressed by the proportion of the area in which the latent images are reproduced. A delicate gradation of the images cannot be expressed or a fine resolution of the line images cannot be reproduced precisely if the particle size of the toner is not uniform. Moreover, when the latent digital images are subjected to reversed development, a lack of the images or irregularity in the image concentration will occur if the distribution of the toner in the magnetic brush formed on the development sleeve is not uniform. Therefore, the fluidity of the developing agent, particularly the delivering property of the developing agent on the development sleeve, should be uniform.
A bias potential imposed on the development sleeve plays as a driving force for the development by the toner in the reversed development method. Since fluctuations in the toner charge will result in the adhesion of the toner to the portions where images are not formed, the amount of the charges on the toner should be controlled precisely.
A photoconductor comprising an organic photoconductive layer has been frequently used for the image forming apparatus in recent years. In this case, a "filming phenomena" (a phenomena in which the toner adheres on the surface of the photoconductor forming a film) tends to occur by repeating development because of a high affinity between the resins forming a photoconductive layer and the toner. Consequently, a toner composition exhibiting improved cleaning performance is required.